Chemo-Mechanical Coupling in the Transport Cycle of a Heme ABC Transporter

Mechanistic insights of ABC importer HutCD involved in heme internalization by Vibrio cholerae

Heme internalization by pathogenic bacteria inside a human host to accomplish the requirement of iron for important cellular processes is of paramount importance. Despite this, the mechanism of heme import by the ATP-binding-cassette (ABC) transporter HutCD in Vibrio cholerae remains unexplored. We have performed biochemical studies on ATPase HutD and its mutants, along with molecular modelling, docking and unbiased all-atom MD simulations on lipid-solvated models of permease-ATPase complex HutCD. The results demonstrated mechanisms of ATP binding/hydrolysis and trapped transient and global conformational changes in HutCD, necessary for heme internalization. ATPase HutD forms a dimer, independent of the permease HutC. Each HutD monomer canonically binds ATP in a 1:1 stoichiometry. MD simulations demonstrated that a rotational motion of HutC dimer occurs synchronously with the inter-dimeric D-loop interactions of HutDs. F151 of TM4–TM5 loop of HutC, packs with ATP and Y15 of HutD, initiating ‘cytoplasmic gate opening’ which mimics an ‘outward-facing’ to ‘inward-facing’ conformational switching upon ATP hydrolysis. The simulation on ‘inward-facing’ HutCD culminates to an ‘occluded’ state. The simulation on heme-docked HutCD indicated that the event of heme release occurs in ATP-free ‘inward-facing’ state. Gradual conformational changes of the TM5 helices of HutC towards the ‘occluded’ state facilitate ejection of heme.

Extracellular haem utilization by the opportunistic pathogen Pseudomonas aeruginosa and its role in virulence and pathogenesis

Iron is an essential micronutrient for all bacteria but presents a significant challenge given its limited bioavailability. Furthermore, iron's toxicity combined with the need to maintain iron levels within a narrow physiological range requires integrated systems to sense, regulate and transport a variety of iron complexes. Most bacteria encode systems to chelate and transport ferric iron (Fe³⁺) via siderophore receptor mediated uptake or via cytoplasmic energy dependent transport systems. Pathogenic bacteria have further lowered the barrier to iron acquisition by employing systems to utilize haem as a source of iron. Haem, a lipophilic and toxic molecule, presents a significant challenge for transport into the cell. As such pathogenic bacteria have evolved sophisticated cell surface signaling (CSS) and transport systems to sense and obtain haem from the host. Once internalized haem is cleaved by both oxidative and non-oxidative mechanisms to release iron. Herein we summarize our current understanding of the mechanism of haem sensing, uptake and utilization in Pseudomonas aeruginosa, its role in pathogenesis and virulence, and the potential of these systems as antimicrobial targets.

Free Energy Analysis of a Conformational Change of Heme ABC Transporter BhuUV-T

The heme ATP-binding cassette (ABC) transporter BhuUV-T of bacterial pathogen Burkholderia cenocepacia is required to transport heme across the inner cell membrane. The current hypothesis is that the binding of two ATPs to the nucleotide-binding domains of the transporter drives the initial steps of the transport cycle in which the empty transport sites are reoriented from the cytosol to the periplasm. Molecular details are missing because the structure of a key occluded intermediate remains hypothetical. Here we perform molecular simulations to analyze the free energy surface (FES) of the first step of the reorientation, namely the transition from an open inward-facing (IF) transport site to an occluded (Occ) conformation. We have modeled the latter structure in silico in a previous study. A simple annealing procedure removes residual bias originating from non-equilibrium targeted molecular dynamics. The calculated FES reveals the role of the ATPs in inducing the IF → Occ conformational change and validates the modeled Occ conformation.